You are doing the right thing;  you are setting appropriate standards by not having sodas in your child's diet.  Providing healthy choices in the home environment is an excellent way to provide 'role model' food choices.  Teaching your child that sodas are not a healthy option will hopefully provide a basis upon which your child can later make his/her own choices.  

Why parents allow their children to drink sodas likely relates to a number of factors, including less attention to the diet of their kids, peer pressure, the 'power of advertising', and general availability (including in school vending machines, movie theaters, and other entertainment venues).

The website provides further discussion and recommendations of healthy diet from the American Academy of Pediatrics.